A door is a movable structure used to open and close off an entrance, typically consisting of a panel that swings on hinges or that slides or rotates inside a space. When open, doors admit ventilation and light. A door may be used to control physical atmosphere within a space so that an interior may be more effectively heated or cooled. Doors also act as a barrier to noise, inclement weather, and physical intrusion into a specifically defined space. In contemporary motor vehicles, besides carrying the above-mentioned functions, doors are frequently reinforced to reduce the degree of deformation in the door structure in case such a door is subjected to energy generated by an external force.